US8467498B2 - Image diagnostic apparatus, image processing apparatus, and program - Google Patents
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- the present invention relates to an image diagnostic apparatus, image processing apparatus, and program which inject a contrast medium into a subject and perform subtraction processing between images before and after the injection of the contrast medium.
- DSA examination of the head of a patient abrupt injection of a contrast medium into the patient may cause lack of oxygen and causalgia.
- This causalgia causes the patient to move, and produces motion artifacts on subtraction images (also called DSA images).
- pixel shifting is manually performed, or semi-automatically performed upon designation of an ROI (Jpn. Pat. Appln. KOKAI Publication No. 2004-112469).
- Motion artifacts appear variously and often change for each frame. In such a case, pixel shifting needs to be performed for each frame. In addition, some patients tend to cause motion artifacts, and hence correction is often performed in many series (images acquired as moving images by one injection of a contrast medium). Under the circumstances, it requires a great deal of labor to perform correction even in automatic pixel shifting.
- an apparatus comprising a radiography unit which generates a mask image and a contrast image before and after injection of a contrast medium, an image storage unit which stores the mask image and the contrast image, a region setting unit which sets a region of interest from the mask image and the contrast image, a pixel shift amount detecting unit which detects a pixel shift amount between the mask image and the contrast image upon localization to the region of interest, and a processing unit which shifts at least one of the mask image and the contrast image in accordance with the detected pixel shift amount and performs subtraction between the images.
- FIG. 1 is a block diagram showing the arrangement of an X-ray diagnostic apparatus according to an embodiment of the present invention
- FIG. 2 is a view showing an example of an initial window for pixel shift processing in this embodiment
- FIG. 3 is a flowchart showing a pixel shift processing procedure in this embodiment
- FIG. 4A is a view showing an example of an ROI detected in ROI identification processing step SO 1 in FIG. 3 ;
- FIG. 4B is a view showing an example of an ROI automatically set with respect to the skull base in ROI identification processing step S 01 in FIG. 3 ;
- FIG. 5 is a view showing another example of the initial window for pixel shift processing in this embodiment.
- FIG. 6 is a view showing a modification of the pixel shift processing procedure in FIG. 3 ;
- FIG. 7 is a graph showing the profile of artifact signal intensity in this embodiment.
- FIG. 8 is a graph showing the profile of an artifact occurrence frequency in this embodiment.
- FIG. 9 is a flowchart showing another modification of the pixel shift processing procedure in FIG. 3 ;
- FIG. 10A is a graph showing an artifact intensity profile Ep(k) in this embodiment.
- FIG. 10B is a graph showing an artifact frequency profile Fp(k) in this embodiment.
- the present invention includes an image diagnostic apparatus which can acquire contrast images and non-contrast images.
- This type of image diagnostic apparatus includes an X-ray diagnostic apparatus, ultrasonic diagnostic apparatus, magnetic resonance imaging apparatus, PET, and SPECT. Any of these apparatuses can be applied to the present invention. The following will exemplify an X-ray diagnostic apparatus.
- FIG. 1 shows the arrangement of an X-ray diagnostic apparatus according to this embodiment.
- a radiography unit (image acquisition unit) 2 includes an X-ray tube 1 and an X-ray detector 3 which face each other through a subject P placed on a bed top 4 .
- a C-arm 5 holds the X-ray tube 1 and the X-ray detector 3 .
- a support mechanism (not shown) supports the C-arm 5 so as to allow it make multiaxial rotation and movement.
- This apparatus uses a flat panel detector (FPD) including a scintillator and a photodiode array as the X-ray detector 3 .
- the apparatus may use a combination of an image intensifier and a TV camera as the X-ray detector 3 .
- An injector (not shown) for automatically injecting a contrast medium into the subject P is placed beside the bed top.
- An image processing apparatus 10 receives an analog video signal output from the X-ray detector 3 through an analog/digital converter (A/D) 13 .
- the image processing apparatus 10 includes a control unit 11 , a filtering processing unit 14 which performs filtering processing such as high-frequency enhancement filtering with respect to the X-ray image data which is generated by the radiography unit 2 and converted into a digital signal by the analog/digital converter 13 , an image memory 15 which stores the X-ray image data before and after filtering processing, an affine transformation unit 16 which performs image enlargement/movement with respect to the X-ray image data, a console 17 , a lookup table (LUT) 18 which performs grayscale conversion, an ROI identifying unit 19 , a pixel shift amount detecting unit 20 , a pixel shift processing unit 21 , a subtraction processing unit 22 , a digital/analog converter (D/A) 23 , and a display 25 .
- filtering processing unit 14 which performs filtering processing such as high-frequency enhancement filtering with respect to the X-ray image data which is generated by the radiography unit 2 and converted into a digital signal by the analog/
- an X-ray image before the injection of the contrast medium, more accurately before the contrast medium flows into a radiographic region, is identified as a mask image
- X-ray images after the injection of the contrast medium, more accurately after the contrast medium flows into the radiographic region are identified as contrast images.
- a contrast image includes both a region (contrast region) mainly occupied by a blood vessel with density being increased by the contrast medium and a region (non-contrast region) where no density change has occurred.
- a mask image substantially includes no contrast region and is occupied by a non-contrast region.
- the ROI identifying unit 19 identifies a processing target region (to be referred to as a region of interest ROI) for the processing (pixel shift amount detection processing) of identifying a moving amount (pixel shift amount) for eliminating the positional shift between a mask image before the injection of contrast medium and a contrast image after the injection of the contrast medium which is mainly caused by the body movement of a subject.
- the pixel shift amount detecting unit 20 detects the positional shift (pixel shift amount) between a contrast image after the injection of a contrast medium and a mask image before the injection of the contrast medium upon localization to the detected region of interest ROI.
- the subtraction processing unit 22 generates a subtraction image by shifting at least one of a mask image and a contrast image in accordance with a pixel shift amount and performing subtraction between the images.
- the display 25 directly receives the subtraction image through the digital/analog converter 23 and displays it.
- contrast medium radiography this apparatus repeats radiography before and after the injection of a contrast medium.
- the contrast medium is injected almost at the same time when the apparatus starts radiography.
- the apparatus generally handles the image of the first frame as a mask image and the images of the subsequent frames as contrast images.
- the apparatus subtracts a mask image from a contrast image.
- the display 25 displays the subtraction image as an image with a contrast region, i.e., a contrast-enhanced blood vessel region, being left and enhanced.
- Injection of a contrast medium more specifically, abrupt injection of a contrast medium for examination of the head of a subject, may cause lack of oxygen and causalgia.
- the apparatus prepares three kinds of modes, i.e., the manual mode, semi-automatic mode, and automatic mode, for pixel shift processing under the control of the control unit 11 in accordance with how much the processing is to be manually performed.
- the button In setting the manual mode, when the operator turns on the manual button written as [Manual] through the console 17 , the button is set in a depressed state. In this state, the manual pixel shift processing function by the console 17 and the pixel shift processing unit 21 is validated. When the operator presses this button again, the button returns to the projected state, thereby invalidating the manual pixel shift processing function. While this mode is valid, when the operator shifts a mask image by operating the up, down, right, and left keys ( ⁇ , ⁇ , ⁇ , and ⁇ ) of the keyboard with a contrast image being fixed, the subtraction processing unit 22 subtracts the mask image shifted by the pixel shift processing unit 21 on the basis of the operation result and the contrast image to generate a subtraction image.
- the apparatus then displays this image on an image area on the display 25 .
- the operator can perform the same operation by pressing the buttons located beside the manual button on the window.
- the apparatus can display a subtraction image on the basis of the operation result.
- the operator presses the execution key written as [Execute] to confirm the shift amount. Operating this execution key will display a frame on the basis of the result of the application of the shift amount confirmed afterward.
- the operator further presses the application key to apply the shift amount to all subsequent frames. Operating this application key will display all frames after the currently display frame on the basis of the result of the application of the shift amount confirmed afterward.
- the button In setting the semi-automatic mode, when the operator turns on the semi-automatic button written as [Semi-Auto], the button is set in the depressed state. In this state, the semi-automatic pixel shift processing function is validated. When the operator presses this button again, the button returns to the projected state, thereby invalidating the semi-automatic pixel shift processing function.
- the apparatus displays a mark indicating a region of interest ROI on a subtraction image first. The operator can select a rectangular shape, a circular shape, or the like for the region of interest ROI. When the operator selects a circular shape as an ROI mark, the apparatus displays the circular ROI mark on the subtraction image.
- the operator can move and enlarge/reduce this ROI mark by a user interface similar to that used for a general graphics drawing function.
- the operator sets the ROI mark so as to contain a region where an artifact has occurred, and presses the execution key to detect a pixel shift amount.
- the apparatus computes correlations while the operator gradually shifts the mask image up, down, left, and right within the region of interest ROI. Identifying the moving amount by which the highest correlation is obtained as an optimal shift amount makes it possible to display the subtraction image on the basis of the shift amount.
- the operator presses the application key. Operating this application key will display all subsequent frame after the currently displayed frame on the basis of the result of the application of the shift amount confirmed afterward.
- FIG. 3 shows a specific flowchart for the processing.
- the apparatus acquires X-ray images of (M+1) frames, and displays M subtraction images by setting one of the frames as a mask image and the M frames as contrast images.
- the mask image is a frame set by default.
- the first frame acquired first after the start of radiography is set as a mask image.
- the subtraction processing unit 22 subtracts a mask image from each of contrast images sequentially read out from the image memory 15 in accordance with the radiography order to sequentially generate subtraction images, and the display 25 consecutively displays the subtraction images.
- the observer presses the automatic button through the console 17 when the subtraction image is displayed. Assume that this frame is the kth frame of the subtraction images.
- the ROI identifying unit 19 identifies the region of interest ROI.
- the pixel shift amount detecting unit 20 identifies a pixel shift amount by processing localized to the region of interest ROI.
- the pixel shift processing unit 21 pixel-shifts the mask image in accordance with the detected pixel shift amount.
- the subtraction processing unit 22 generates a subtraction image by subtraction between a contrast image and the pixel-shifted mask image. The following will describe the contents of each step in detail.
- step S 00 the control unit 11 determines, in accordance with an instruction input by the operator through the console 17 , whether to apply pixel shift processing.
- the control unit 11 performs the following processing.
- the control unit 11 Upon determining on the basis of the above result that Ep(i, j)>E, the control unit 11 registers the corresponding pixel as a target region.
- E is a predetermined threshold.
- the ROI identifying unit 19 calculates, as the artifact intensity Ep(i, j), the total sum of the pixel values of pixels appearing with reverse polarity (typically, positive polarity including zero) with respect to a contrast medium (negative polarity) throughout the M subtraction images.
- the ROI identifying unit 19 determines a pixel group whose artifact intensity Ep(i, j) exceeds the threshold E as the initial (primary) region of interest ROI.
- Most of pixels appearing with reverse polarity to pixels based on the contrast medium after subtraction processing are generated due to the body movement of the subject. The body movement generates pixels with the same polarity as that of the contrast medium and pixels with reverse polarity to the contrast medium.
- the control unit 11 removes a region located outside a predetermined range from the primarily determined region of interest ROI, thereby generating a secondary region of interest ROI. This is because a region of interest in a clinical point of view is generally located near the center of an image, and the occurrence of a motion artifact in the region poses the most serious problem.
- a 0.7N ⁇ 0.7N region in the center of an entire image is set as a target region, as shown in FIG. 4A .
- a region where the target region overlaps the primary region of interest ROI is set as the secondary region of interest ROI.
- the secondary region of interest ROI is determined by removing an annular region other than the central 0.7N ⁇ 0.7N region from the primary region of interest ROI.
- the control unit 11 determines a final region of interest ROI by expanding the secondary region of interest ROI up, down, left, and right to a predetermined range (expansion processing).
- the final region of interest ROI will be simply referred to as the region of interest ROI.
- the control unit 11 performs expansion processing because it is highly probable that there is information associated with an artifact near a region where the artifact has occurred (for example, there is an artifact with negative polarity (based on the contrast medium) near an artifact region with positive polarity (not based on the contrast medium), or an artifact is underevaluated by a threshold process).
- an artifact region For example, there is an artifact with negative polarity (based on the contrast medium) near an artifact region with positive polarity (not based on the contrast medium), or an artifact is underevaluated by a threshold process.
- the control unit 11 sets, as the center of the region of interest ROI, a position separated from the image center by a predetermined distance ⁇ P (a predetermined number of pixels) in the direction from the vertex to the jaw, typically in the negative Y-axis direction.
- ⁇ P a predetermined number of pixels
- the pixel shift amount detecting unit 20 repeatedly computes correlations between the mask image and the contrast images by finely moving the mask image within a search region in a predetermined range from the same position on the mask image upon localizing to the detected region of interest ROI.
- the pixel shift amount detecting unit 20 determines the position where the lowest computation result (correlation coefficient) is obtained as a corresponding position, and stores a moving vector to the position as pixel shift amount data. In this case, correlation computation can be written as
- Ck(i, j) and M(i+ ⁇ i, j+ ⁇ j) are respectively the contrast image and mask image of the kth frame
- N is the matrix size of the image
- ( ⁇ i, ⁇ j) is a shift vector
- CR( ⁇ i, ⁇ j) is a correlation computation result.
- the pixel shift amount detecting unit 20 obtains correlation computation results while shifting ⁇ i and ⁇ j between ⁇ and ⁇ , and detects, as a positional shift, a shift vector by which the correlation computation result is minimized.
- the range of correlation computation is defined by ⁇ and ⁇ , and a step in computation (step of ⁇ i and ⁇ j) is represented by ⁇ .
- r(x) is defined as follows:
- r ⁇ ( x ) ⁇ x ⁇ ⁇ 0 ⁇ : ⁇ ⁇ within ⁇ ⁇ ROI o ⁇ : ⁇ ⁇ outside ⁇ ⁇ ROI ( 4 )
- step S 03 the pixel shift processing unit 21 shifts the mask image by the vector ( ⁇ i 0 , ⁇ j 0 ).
- subtraction step S 04 the control unit 11 subtracts a contrast image Ck(i, j) and a shifted mask image M(i+ ⁇ i 0 , j+ ⁇ j 0 ). The control unit 11 sequentially displays subtraction images.
- the control unit 11 When the observer approves the result and presses the execution button, the control unit 11 registers ( ⁇ i 0 , ⁇ j 0 ) as an optimal pixel shift amount for the kth frame in a region attached to the subtraction image. When displaying the same image next, the control unit 11 uses the result obtained by the registered pixel shift amount and displaying it. When the observer does not approve the result and presses the cancellation button, the control unit 11 deletes the result (S 07 ). When the observer is to apply the same pixel shift amount to the succeeding frame upon approval of the result (S 06 ), he/she presses the application button in step S 05 .
- the above embodiment includes the automatic button. Even if, however, a region of interest ROI is set as an entire image with a semi-automatic button, it suffices to apply the algorithm for automatically setting a region of interest ROI, as shown in FIG. 5 .
- FIG. 6 shows a flowchart for a processing procedure based on a second modification.
- the apparatus acquire (M+1) images, and displays M subtraction images by setting one of the frames as a mask image and the M frames as contrast images.
- the mask image is a frame set by default.
- the first frame is set as a mask image.
- a region of interest ROI is identified by using the M subtraction images.
- the apparatus detects a pixel shift amount for each contrast image, pixel-shifts the mask image on the basis of the detected shift amount, and executes subtraction between the contrast image and the corrected mask image.
- the apparatus performs this operation for each of the contrast images of all the M frames. The contents of each step will be described in detail next.
- control unit 11 obtains the artifact intensity Ep(i, j) and an artifact frequency Fp(i, j) by using the M generated subtraction images in the following manner.
- f ⁇ ( x ) ⁇ x ⁇ : ⁇ ⁇ x ⁇ 0 0 ⁇ : ⁇ ⁇ x ⁇ 0 ( 7 )
- g ⁇ ( x ) ⁇ 1 ⁇ : ⁇ ⁇ x ⁇ 0 0 ⁇ : ⁇ ⁇ x ⁇ 0 ( 8 )
- control unit 11 obtains the artifact intensity Ep(i, j) by calculating total sum of the pixel values of pixels appearing with reverse polarity (typically, positive polarity including zero) to a contrast medium (negative polarity) throughout the M subtraction images.
- reverse polarity typically, positive polarity including zero
- contrast medium negative polarity
- control unit 11 obtains the artifact frequency Fp(i, j) by obtaining the frequency (the number of pixels) at which the pixel value of a pixel appearing with positive polarity exceeds a predetermined threshold for each pixel with respect to the M subtraction images between the mask image and the M contrast images.
- the control unit 11 determines, as the region of interest ROI, a pixel group which satisfies Ep(i, j)>E and Fp(i, j)>F on the basis of the above result.
- E and F are predetermined thresholds.
- the following is an example of a combination of a means for calculating the total sum of positive pixel values and a means for calculating the number of pixels (frequency) each which exceeds a threshold.
- the means for calculating the frequency of pixels each of which exceeds a threshold may be combined with a means for calculating the maximum absolute value of a positive or negative signals for each pixel instead of the means for calculating the total sum of positive signals. Furthermore, it suffices to singly use one of the three means.
- a region outside a predetermined range inside the region of interest ROI is deleted from a registered region. This is because a target region in a clinical point of view is generally located near the center of an image, and the occurrence of a motion artifact in the region poses the most serious problem.
- a region other than the 0.7N ⁇ 0.7N region of the entire image is set as a region outside a target region, and an artifact region outside the range is deleted from the region.
- the control unit 11 further expands the registered region of interest ROI up, down, left, and right to a predetermined range. This is because it is highly probable that there is information associated with an artifact near a region where the artifact has occurred (for example, there is an artifact with negative polarity near an artifact region with positive polarity, or an artifact is underevaluated by a threshold process).
- an artifact region within a radius of 10 pixels, the arbitrary pixel is regarded as an artifact region.
- the control unit 11 repeats the processing from pixel shift amount detection step S 13 to subtraction processing step S 15 for each contrast image through variable N initialization step S 12 , variable N increment step S 16 , and step S 17 of determining whether the variable N has reached a contrast image count M.
- the control unit 11 sequentially performs correlation computation while finely moving the mask image within a search region in a predetermined range from the same position on an image.
- the control unit 11 determines a position where the lowest computation result is obtained as a corresponding position, and stores a moving vector to the position. In this case, correlation computation can be written as
- Ck(i, j) and M(i+ ⁇ i, j+ ⁇ j) are respectively the contrast image of the kth frame and mask image of the kth frame
- N is the matrix size of the image
- ( ⁇ i, ⁇ j) is a shift vector
- CR( ⁇ i, ⁇ j) is a correlation computation result.
- the control unit 11 obtains correlation computation results while shifting ⁇ i and ⁇ j between ⁇ and ⁇ , and detects, as a positional shift, a shift vector by which the correlation computation result is minimized.
- the range of correlation computation is defined by ⁇ and ⁇ , and a step in computation (step of ⁇ i and ⁇ j) is represented by ⁇ .
- r(x) is defined as follows:
- r ⁇ ( x ) ⁇ x ⁇ ⁇ 0 ⁇ : ⁇ ⁇ within ⁇ ⁇ ROI o ⁇ : ⁇ ⁇ outside ⁇ ⁇ ROI ( 10 )
- ( ⁇ i 0 , ⁇ j 0 ) is a shift vector by which CR( ⁇ i, ⁇ j) is minimized.
- the control unit 11 shifts the mask image by the vector ( ⁇ ik, ⁇ jk).
- the control unit 11 performs subtraction between Ck(i, j) and a corrected mask image M(i+ ⁇ ik, j+ ⁇ jk). The control unit 11 sequentially displays subtraction results.
- the control unit 11 registers an optimal shift ( ⁇ ik, ⁇ jk) for each frame in a region attached to the subtraction image (S 19 ).
- the automatic button then returns to the initial state.
- the control unit 11 uses the result obtained by the registered pixel shift amount and displays it.
- the result is reset (S 20 ).
- the control unit 11 obtains a contrast medium injection signal intensity Em(i, j) and a contrast medium injection signal frequency Fm(i, j) by using M generated subtraction images as follows:
- INVf ⁇ ( x ) ⁇ 0 ⁇ : ⁇ ⁇ x ⁇ 0 x ⁇ : ⁇ ⁇ x ⁇ 0 ( 13 )
- INVg ⁇ ( x ) ⁇ 0 ⁇ : ⁇ ⁇ x ⁇ 0 1 ⁇ : ⁇ ⁇ x ⁇ 0 ( 14 )
- the control unit 11 Upon determining on the basis of the above result that Em(i, j) ⁇ G and Fm(i, j)>H, the control unit 11 registers the corresponding pixel as a contrast-medium-injected region.
- G and H are predetermined thresholds.
- a contrast medium injected regions are excepted from the 0.7N ⁇ 0.7N region centered in the entire image. The remained region is registered as a region of interest ROI. Note that this embodiment has exemplified the combination of the means for calculating the total sum of negative signals and the means for calculating the frequency of pixels each which exceeds a threshold.
- the means for calculating the frequency of pixels each of which exceeds a threshold may be combined with a means for calculating the minimum value projection of a negative signal for each pixel instead of the means for calculating the total sum of negative signals. Furthermore, it suffices to singly use one of the three means.
- a region of interest ROI is identified from one of an artifact occurrence region and a contrast-medium-injected region.
- a region of interest ROI may be identified from a combination of them. More specifically, a contrast-medium-injected region is excluded from an artifact occurrence region.
- the control unit 11 calculates a pixel shift amount. Calculating an optimal pixel shift amount for each frame makes it possible to perform correction with very high accuracy. This technique, however, greatly increase the calculation amount and may not be useful for a clinic case requiring high speed.
- the control unit 11 therefore calculates an optimal pixel shift for each frame group including a plurality of frames as a unit instead of calculating an optimal pixel shift amount for each frame.
- the control unit 11 obtains an artifact signal intensity for each frame as follows:
- f ⁇ ( x ) ⁇ x ⁇ : ⁇ ⁇ x ⁇ 0 0 ⁇ : ⁇ ⁇ x ⁇ 0 ( 17 )
- g ⁇ ( x ) ⁇ 1 ⁇ : ⁇ ⁇ x ⁇ 0 0 ⁇ : ⁇ ⁇ x ⁇ 0 ( 18 )
- FIGS. 7 and 8 show the profiles of an artifact signal intensity and artifact occurrence frequency.
- Ep and Fp be intervals at which frame groups are sorted, frames up to an earlier one of the first frame at which the error with respect to Ep( 1 ) exceeds Ep and the first frame at which the error with respect to Fp( 1 ) exceeds Fp are determined as a frame group.
- the sth frame is a frame at which the above error exceeds Ep or Fp
- frames up to an earlier one of the first frame at which the error with respect to Ep(s) exceeds Ep and the first frame at which the error with respect to Fp(s) exceeds Fp are determined as the next frame group.
- the control unit 11 determines a frame group by using both an artifact signal intensity and an artifact occurrence frequency. However, it suffices to determine a frame group by using one of them.
- the control unit 11 determines frames ranging from the first frame to the (s ⁇ 1)th frame as a single frame group. Assume that of the artifact signal intensity E and the artifact occurrence frequency F, the artifact signal intensity exceeds the threshold first. In this case, letting r be a frame nearest to the average or median of Ep( 1 ) and Ep(s ⁇ 1), the control unit 11 applies an optimal pixel shift amount ( ⁇ ir, ⁇ ir) for the rth frame to the mask image, and subtracts it from the contrast images ranging from the first frame to the (s ⁇ 1)th frame. In this case, the average functions to reduce errors in all the frames. In general, however, an artifact often changes abruptly, using the median may reduce artifacts more in terms of the overall moving image.
- the control unit 11 applies an optimal pixel shift amount ( ⁇ iu, ⁇ ju) at the uth frame to the mask image and subtracts the mask image from the contrast images of the sth frame to the (t ⁇ 1)th frame.
- frame groups are sorted by using the artifact signal intensity E and the artifact occurrence frequency F.
- This method has a merit of a small calculation amount. Assume that after a shift has occurred to, for example, the right, an abrupt shift has occurred in the opposite direction by an amount twice that of the preceding shift. In this case, although the occurrence position of the artifact has actually changed, it can be regarded that no change has occurred in terms of the total amount of artifacts. In order to solve this problem, it is preferable to also perform calculation on the basis of the distribution of artifacts.
- Dp is the difference value between artifact distributions of two images, and represents the total sum of positive and zero absolute values excluding negative values.
- Dp Letting Dp be intervals at which frame groups are sorted, frames up to the first frames(s) at which the error with respect to Dp( 1 ) exceeds Dp are determined as a frame group. With the above processing, frames ranging from the first frame to the (s ⁇ 1)th frame are determined as a frame group. If the vth frame is nearest to the median of Dp( 1 ) and Dp(s ⁇ 1), the control unit 11 applies an optimal pixel shift amount ( ⁇ iv, ⁇ jv) at the vth frame to the mask image, and subtracts the mask image from the contrast images of the first frame to the (s ⁇ 1)th frame. (Seventh Modification)
- a pixel shift amount at an average frame of a frame group is set as a typical pixel shift amount.
- Postprocessing can be expected to obtain an effect of smoothing artifacts.
- this technique cannot determine a pixel shift amount up to, for example, the sth frame in the first frame group, and hence is very poor in real time performance. For this reason, when high real time performance is required, it is preferable to handle a pixel shift amount at the first frame in a frame group as a typical pixel shift amount.
- a frame group is uniquely determined. This technique, however, needs to permit artifacts to some extent. On the other hand, processing for each frame instead of each frame group requires much calculation time. As a compromise between these techniques, the present invention may use a technique of performing processing for a frame group first, and then dividing the frame group upon completion of the processing.
- the control unit 11 divides the frame group of the frame groups at the first stage into two parts. Since the first frame group ranges from the first frame to the (s ⁇ 1)th frame, the control unit 11 divides the frame group into a frame group ranging from the first frame to the ⁇ (s ⁇ 1)/2 ⁇ th frame and a frame group ranging from the ⁇ (s ⁇ 1)/2+1 ⁇ th frame to the (s ⁇ 1)th frame, and recalculates optimal pixel shift amounts for the respective frame groups. The control unit 11 pixel-shifts the mask image on the basis of the recalculated pixel shift amount and executes subtraction, thereby sequentially reflecting the results in display in the order of completion.
- the control unit 11 processes the second frame group in the same manner as described above. Upon completing the processing of all the frame groups at the second stage, the control unit 11 further divides the second frame group. The control unit 11 then processes the third frame group in the same manner as described above. The control unit 11 continues this processing up to the frame unit. Note that the control unit 11 may terminate the processing when at least one of a minimum artifact intensity Epmin, a minimum artifact frequency Fpmin, and Dpmin or a combination thereof is satisfied.
- FIG. 9 shows an operation procedure in this modification.
- the observer determines the start of pixel shifting.
- the start of this operation is automatically determined with respect to a radiographed image
- This modification includes a switch for determining during the execution of a radiography program whether to automatically perform pixel shifting. If this switch is turned on, the following processing is performed. If the switch is turned off, normal processing is performed.
- the program obtains an artifact intensity Ep(k) and an artifact frequency Fp(k) for each frame by using the M subtraction images.
- the program performs a threshold process with respect to each of Epmin and Fpmin for the obtained calculated values.
- the program determines that pixel shifting is required for a frame which exceeds at least one of the thresholds ( FIGS. 10A and 10B explain a case wherein only an artifact intensity is used).
- the program determines by using both the artifact intensity and the artifact frequency whether pixel shifting is required.
- the necessity/unnecessity of pixel shifting may be determined by using one of them.
- processing is performed in the same manner as in the second modification. However, this processing is performed for only a frame for which necessity of pixel shifting is determined. If there is no frame for which necessity of pixel shifting is determined, the processing is not performed.
- the automatic button is set in the depressed state, and the execution, cancellation, and application buttons are set in the gray-out state. After the completion of the processing, the execution, cancellation, and application buttons return to the initial states.
- the observer determines that the processing result is proper, and presses the execution button, the processing is confirmed, and a pixel shift amount is registered. As a result, the automatic button returns to the initial state. The subsequent display operation is performed by using the calculated pixel shift amount. If the processing is not required, the automatic button automatically returns to the initial state.
- the processing is automatically applied.
- the necessity/unnecessity of correction is determined by the automatic method described in the second and third embodiment (modification), and the processing is executed in the background.
- the observer determines whether to apply this. More specifically, the automatic button in the pixel shift column is set in the initial projected state, and the same processing as that in the third modification is executed in the background up to the identification of a pixel shift amount (pixel shifting and subtraction are not executed).
- the control unit 11 executes pixel shifting and subtraction on the basis of the results calculated so far. The operator confirms the processing by using the execution button.
- pixel shifting is automatically performed.
- the processing for this frame is omitted from the automatic processing, and priority is given to the processing result determined by the observer.
- constituent elements can be variously modified and embodied at the execution stage within the spirit and scope of the invention.
- Various inventions can be formed by proper combinations of a plurality of constituent elements disclosed in the above embodiments. For example, several constituent elements may be omitted from all the constituent elements in each embodiment. In addition, constituent elements of the different embodiments may be combined as needed.
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Abstract
Description
E p(i,j)=f[Sk(i,j)] (1)
where Sk(i, j) is the subtraction image of the kth frame, and f(x) is defined as follows:
where Ck(i, j) and M(i+Δi, j+Δj) are respectively the contrast image and mask image of the kth frame, N is the matrix size of the image, (Δi, Δj) is a shift vector, and CR(Δi, Δj) is a correlation computation result. The pixel shift
where Sk(i, j) is the subtraction image of the kth frame, and S is a predetermined constant which discriminates an artifact from noise in Sk(i, j). Then f(x) and g(x) are defined as follows:
where Ck(i, j) and M(i+Δi, j+Δj) are respectively the contrast image of the kth frame and mask image of the kth frame, N is the matrix size of the image, (Δi, Δj) is a shift vector, and CR(Δi, Δj) is a correlation computation result. The
where Sk(i, j) is the subtraction image of the kth frame, and S is a predetermined constant which discriminates a contrast medium injection signal from noise in Sk(i, j). Then, INVf(x) and INVg(x) are defined as follows:
Sk(i, j) is the subtraction image of the kth frame, and S is a predetermined constant which discriminates an artifact from noise in Sk(i, j). Then, f(x) and g(x) are defined as follows:
where Dp is the difference value between artifact distributions of two images, and represents the total sum of positive and zero absolute values excluding negative values. Letting Dp be intervals at which frame groups are sorted, frames up to the first frames(s) at which the error with respect to Dp(1) exceeds Dp are determined as a frame group. With the above processing, frames ranging from the first frame to the (s−1)th frame are determined as a frame group. If the vth frame is nearest to the median of Dp(1) and Dp(s−1), the
(Seventh Modification)
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